Generally, there is a need to obtain data from the implanted components of a cochlear implant. Such data collection enables detection and confirmation of the normal operation of the device, and allows stimulation parameters to be optimized to suit the needs of individual recipients. This includes data relating to the response of the auditory nerve to stimulation.
Typically, following the surgical implantation of a cochlear implant, the implant is fitted or customized to conform to the specific recipient demands. This involves the collection and determination of patient-specific parameters such as threshold levels (T levels) and maximum comfort levels (C levels) for each stimulation channel. The procedure is performed manually by applying stimulation pulses for each channel and receiving an indication from the implant recipient as to the level and comfort of the resulting stimulation.
One method of interrogating the performance of an implanted cochlear implant and making objective measurements of patient-specific data such as T and C levels is to directly measure the response of the auditory nerve to an electrical stimulus. The direct measurement of neural responses, commonly referred to as Electrically Evoked Compound Action Potentials (ECAPs) in the context of cochlear implants, provides an objective measurement of the response of the nerves to electrical stimulus. Following electrical stimulation, the neural response is caused by the superposition of single neural responses at the outside of the axon membranes. The measured neural response is transmitted to a system located external to the CI recipient, typically via a telemetry system. This provides measurements of the ECAPs from within the cochlea in response to various stimulations. Generally, the neural response resulting from a stimulus presented at one electrode is measured at a neighboring electrode, although this need not be the case.
A prior art method for obtaining ECAP's is illustrated with reference to FIG. 1 and FIG. 2.
In FIG. 1 there is shown an ECAP(t) plot 1 of a sequence of measurements containing seven ECAP measurements 2 through 8 which show a clearly distinguishable neural response. Thus, each measurement waveform 2 through 8 comprises a clearly distinctive negative peak (N1) 10 and a clearly distinctive positive peak (P1) 9. Only one positive and negative peak is shown in FIG. 1 for clarity. The parameter in the plot of FIG. 1 is the neural stimulus, for instance stimulus current level (see FIG. 2). The strongest neural stimulus is used to obtain the response 2 and the weakest neural stimulus is used to obtain the response 8.
The measurement waveforms toward the top of the graph depicted in FIG. 1 (measurement waveforms 2 and 3, for example) indicate a stronger neural response to a relatively large neural stimulus, while the measurement waveforms toward the bottom of the graph (measurement waveforms 7 and 8, for example) indicate a weaker neural response with reduced neural stimuli strengths. As it clearly appears from FIG. 1, a progressively more distinct neural response, i.e. a neural response showing distinct peaks (such as 9) and dips (such as 10) is obtained for progressively more powerful neural stimuli. At weak neural stimuli, it becomes progressively more difficult to determine if a neural response is in fact elicited.
Distinguishing between measurements that display a neural response such as those of FIG. 1 and measurements which do not display a neural response is an important aspect of performing such measurements. This task can be extremely difficult, for instance when the combination of stimulus artefact and noise gives the appearance of a weak neural response.
In particular, the minimum stimulus current level required to evoke a neural response at a given electrode in a cochlear implant is referred as the threshold level (for that particular electrode, i.e. for the frequency corresponding to the placement of this electrode in the patients cochlear). In general, the threshold level profiles are correlated with MAP T and C profiles, and thus threshold levels can be used as a guide for MAP fitting. Accordingly, accurate determination of threshold level values for each electrode and for each recipient is highly desirable.
One conventional approach to determine the threshold level values is the Amplitude Growth Function (AGF) method. The AGF method is based on the premise that the peak-to-peak amplitude of a neural response increases linearly with stimulus current level. However, the relationship is more accurately defined by a sigmoidal function. By obtaining the peak-to-peak amplitude value at different stimulus current levels, a regression line may be drawn through these measurement points and extrapolated to the point at which the peak-to-peak amplitude becomes zero, thus indicating the threshold stimulus level.
For example, FIG. 2 illustrates a typical prior art measurement plot 11 of peak-to-peak amplitude ECAPs (in microvolts) vs. stimulus current level (in digitized current level units). The threshold value corresponds to the amplitude 0 V and is indicate by the line 14 in FIG. 2. In FIG. 2, the current level scale shown extends from 96 to 192 current level units with each unit representing an increasingly lager current. The individual measurement results are indicated by the dots, such as 12 in the plot 11. Typically, the measurements can be fitted with a number of regression lines, but in the example shown in FIG. 2 the regression 13, yielding a possible threshold value of 148 current level units is shown. As it appears from FIG. 2, the regression line 13 intercepts the x-axis (the current level axis) at a point 15.
Prior art methods of establishing the neural excitation threshold suffers from a number of disadvantages of which some are listed below:
(1) Typically, the AGF approach requires a significant number of measurements above the threshold to enable a regression line to be determined.
(2) An alternative method of threshold determination is visual detection. The visual-detection method is more subjective, where threshold is determined as the lowest current level for which an ECAP waveform can be visually identified by a human observer. Unlike the linear regression method which requires multiple responses, the visual detection method utilizes a single response. This method works best in systems with a low noise floor. For systems with a high noise floor, ECAP responses occurring at low current levels may be obscured, resulting in artificially elevated threshold estimates. Visual detection depends critically on the acuity of the observer to distinguish between neural responses and artefact or noise. Visual detection of threshold is also observer-dependent.
(3) ECAP diagnostics often require significant clinical time. Existing data collection methods do not leverage pre-existing patient data during follow-up appointments and/or rechecking objective measures.
It is consequently desirable to provide a method and system that significantly reduces the number of supra-threshold level measurements required at a later recording session for obtaining the Evoked Compound Action Potential Recordings in patients provided with a cochlear implant.
It is further desirable to provide a method of as stated above that can be carried out significantly faster than known prior art methods.